1. Field of the Invention
The present invention relates to a driving circuit for a light emitting diode.
2. Description of the Related Art
In recent years, as a backlight of a liquid crystal panel or as an illumination device, a light emitting apparatus is employed, which is configured using an LED (light emitting diode). FIG. 1 is a circuit diagram which shows a typical configuration of a light emitting apparatus. A light emitting apparatus 1003 includes multiple LED strings 1006_1 through 1006_n, a switching power supply 1004, and a current driving circuit 1008.
Each LED string 1006 includes multiple LEDs connected in series. The switching power supply 1004 boosts an input voltage Vin, and supplies a driving voltage Vout to one terminal of each of the LED strings 1006_1 through 1006_n.
The current driving circuit 1008 includes current sources CS1 through CSn which are respectively provided to the LED strings 1006_1 through 1006_n. Each current source CS supplies, to the corresponding LED string 1006, a driving current ILED that corresponds to the target luminance level.
The switching power supply 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 controls the on/off duty ratio of the switching transistor M1 so as to adjust the driving voltage Vout.
With such a light emitting apparatus 1003, in some cases, in order to adjust the luminance level of each LED string 1006, a PWM (Pulse Width Modulation) control operation is performed on the on period TON and the off period TOFF of the driving current ILED. Such a control operation is also referred to as the “burst dimming control operation” or “burst driving control operation”. Specifically, a burst controller 1009 of the current driving circuit 1008 receives pulse signals PWM1 through PWMn each having a duty ratio that corresponds to the luminance level so as to perform a switching control operation on the respective current sources CS1 through CSn.